Some commentators have argued that there is no room in Aristotle's natural science for simple, or unconditional, physical necessity, for the only necessity that governs all natural substances is hypothetical and teleological. Against this view I argue that, according to Aristotle, there are two types of unconditional physical necessity at work in the material elements, the one teleological, governing their natural motions, and the other non-teleological, governing their physical interaction. I argue as well that these two types of simple necessity also govern everything made out of the elements, that is, all other natural substances and artifacts.